1. Field of the Invention
This invention relates to a method and an apparatus for reduction-projection type mask and wafer alignment using a projection type exposure technique in the field of manufacture of semiconductor devices, such as IC or LSI.
2. Description of the Prior Art
Generally, the reduction-projection lens 2 for projecting the pattern 1' of the mask 1 onto the wafer 12 as shown in FIG. 1 can be used only with light having a single wavelength in its specification. If light at another wavelength (for example, the mercury e-line which is insensitive to photoresists) is used for alignment, the resolution of the image is deteriorated, and forms an image at a different point from that formed by the exposure light (at the wave length of the mercury g-line 3) due to chromatic aberration. In the conventional method using the mercury e-line 4 for alignment and the mercury g-line 3 (or h- and/or g-line) for exposure, a fine adjustment device is required to correct this vertical displacement on the wafer due to chromatic aberration. The vertical fine adjustment by the fine adjustment device, however, involves an undesired lateral displacement after alignment. In order to eliminate this lateral displacement due to chromatic aberration, a lens having the same focal point for the wavelength of both the mercury e-line and g-line (or h- and/or g-line) has recently been developed, and this lens has an image-reduction ratio of approximately 1/1. However, a comparable lens having a reduction ratio of 1/10 has not been developed as yet for the purpose of optically printing minute patterns on the wafer. In the conventional methods, therefore, optical printing of minute patterns on the wafer by the use of a projection lens requires the fine adjustment device as mentioned above.
When a circuit pattern is optically printed by means of a lens with the image-forming ration of 1/1, a minute pattern less than 5.mu.in size cannot be projected with high resolution. For this reason, a circuit pattern smaller than a certain size has been incapable of being optically printed on the wafer by 1/1 projection. When the circuit pattern formed on the mask is projected onto the wafer through a reduction-projection lens, the resolution is increased. By this method, a very minute pattern of about 1.mu. in size can be optically printed on the wafer. In the manufacture of a semiconductor inetegrated circuit on the wafer, however, about 10 masks must be overlaid sequentially on progressively formed circuit patterns. This requires a highly sophisticated technique for each alignment between the mask and the wafer.